A voltage-controlled oscillator (VCO) is a major component in a wireless communication system such as a mobile phone, a wireless network card, etc. The VCO may cooperate with components including a phase frequency detector, a charge pump, a loop filter, a frequency divider, etc., to form a radio frequency integrated circuit. For mobile communications, it is important to improve the performance, as well as to reduce an occupied area and costs of the VCO.
Referring to FIG. 1, a conventional VCO includes two inductors (Ld) and two varactors (Cd1) that cooperatively form an LC tank 7, and two transistors (M) that cooperatively form a cross-connected pair 8. The cross-connected pair 8 is connected between the LC tank 7 and ground. Each varactor (Cd1) has a capacitance variable according to a control voltage (Vt). The control voltage (Vt) may be adjusted to change the capacitances of the varactors (Cd1), thereby adjusting a frequency of a differential output signal pair generated by the conventional VCO.
The conventional VCO has the following drawbacks:
1. The inductors (Ld) occupy a relatively large area, which is adverse in reducing the occupied area and the costs of the conventional VCO.
2. Only the capacitances of the varactors (Cd1) are variable, and therefore an adjustable range of the output frequency is relatively narrow.
3. Only one differential output signal pair is provided, and therefore the conventional VCO is not suitable for a system that operates based on multiple differential output signal pairs with different frequencies.